1. Field of Invention
This invention relates to voltage conversion circuits. More particularly, this invention relates to DC-AC voltage transformation circuits wherein a resonating piezoelectric transformer is used, along with complementary circuit components, to efficiently convert a DC first voltage to a transformer-output AC second voltage.
2. Description of Prior Art
Wire wound-type electromagnetic transformers have been used for generating high voltage in internal power circuits of devices such as televisions or fluorescent lamp ballasts. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce.
To remedy this and many other problems of the wire-wound transformer, piezoelectric transformers utilizing the piezoelectric effect have been provided in the prior art. In contrast to the general electromagnetic transformer, the piezoelectric ceramic transformer has a number of advantages. The size of a piezoelectric transformer can be made smaller than electromagnetic transformers of comparable transformation ratio. Piezoelectric transformers can be made nonflammable, and they produce no electromagnetically induced noise.
The ceramic body employed in prior piezoelectric transformers take various forms and configurations, including rings, flat slabs and the like. A typical example of a prior piezoelectric transformer is illustrated in FIG. 1. This type of piezoelectric transformer is commonly referred to as a "Rosen-type" piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 to Rosen, and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type piezoelectric transformer comprises a flat ceramic slab 210 which is appreciably longer than it is wide and substantially wider than thick. As shown in FIG. 11, a piezoelectric body 210 is employed having some portions polarized differently from others. In the case of FIG. 11, the piezoelectric body 210 is in the form of a flat slab that is considerably wider than it is thick, and having greater length than width. A substantial portion of the slab 210, the portion 212 to the right of the center of the slab is polarized longitudinally, whereas the remainder of the slab is polarized transversely to the plane of the face of the slab. In this case the remainder of the slab is actually divided into two portions, one portion 214 being polarized transversely in one direction, and the remainder of the left half of the slab, the portion 216 also being polarized transversely but in the direction opposite to the direction of polarization in the portion 214.
In order that electrical voltages may be related to mechanical stress in the slab 210, electrodes are provided. If desired, there may be a common electrode 218, shown as grounded. For the primary connection and for relating voltage at opposite faces of the transversely polarized portion 214 of the slab 210, there is an electrode 220 opposite the common electrode 218. For relating voltages to stress generated in the longitudinal direction of the slab 210, there is a secondary or high-voltage electrode 222 cooperating with the common electrode 218. The electrode 222 is shown as connected to a terminal 224 of an output load 226 grounded at its opposite end.
In the arrangement illustrated in FIG. 1, a voltage applied between the electrodes 218 and 220 is stepped up to a high voltage between the electrodes 218 and 222 for supplying the load 226 at a much higher voltage than that applied between the electrodes 218 and 220.
An inherent problem of such prior piezoelectric transformers that they have relatively low power transmission capacity. This disadvantage of prior piezoelectric transformers relates to the fact that little or no mechanical advantage is realized between the driver portion of the device and the driven portion of the device, since each is intrinsically a portion of the same electroactive member. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices. Additionally, even under resonant conditions, because the piezoelectric voltage transmission function of Rosen-type piezoelectric transformers is accomplished by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x'-y' surface areas) of the piezoelectric member, which changes are of relatively low magnitude, the power handling capacity of prior circuits using such piezoelectric transformers is inherently low.
In addition, because the typical prior piezoelectric transformer accomplishes the piezoelectric voltage transmission function by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x'-y' surface areas) of the piezoelectric member, it is generally necessary to alternately apply positive and negative voltages across opposing faces of the "driver" portion of the member in order to "push" and "pull", respectively, the member into the desired shape. Even under resonant conditions, prior electrical circuits that incorporate such prior piezoelectric transformers are relatively inefficient, because the energy required during the first half-cycle of operation to "push" the piezoelectric member into a first shape is largely lost (i.e. by generating heat) during the "pull" half-cycle of operation. This heat generation corresponds to a lowering of efficiency of the circuit, an increased fire hazard, and/or a reduction in component and circuit reliability. Furthermore, in order to reduce the temperature of such heat generating circuits, the circuit components (typically including switching transistors and other components, as well as the transformer itself) are oversized, which reduces the number of applications in which the circuit can be utilized, and which also increases the cost/price of the circuit.
Because the power transmission capacity of such prior piezoelectric transformers is so low, it has become common in the prior art to combine several such transformers together into a multi-layer "stack" in order to achieve a greater power transmission capacity than would be achievable using one such prior transformer alone. This, of course, increases both the size and the manufacturing cost of the transformer; and the resulting power handling capacity of the "stack" is still limited to the arithmetic sum of the power handling capacity of the individual elements.
Accordingly, it would be desirable to provide a piezoelectric transformer design that has a higher power transmission capacity than similarly sized prior piezoelectric transformers.
It would also be desirable to provide a piezoelectric transformer that is smaller than prior piezoelectric transformers that have similar power transmission capacities.
It would also be desirable to provide a piezoelectric transformer in which the "driver" portion of the device and the "driven" portion of the device are not the same electro-active element.
It would also be desirable to provide a piezoelectric transformer that develops a substantial mechanical advantage between the driver portion of the device and the driven portion of the device.
It would also be desirable to provide a piezoelectric transformer that, at its natural frequency, oscillates with greater momentum than is achievable with comparably sized prior piezoelectric transformers.
U.S. patent application Ser. No. 08/864,029 filed May 27, 1997, (now U.S. Pat. No. 5,834,882) which is included by reference thereto, describes a multi-layered, laminated, piezoelectric transformer that has demonstrated an ability to convert a primary or input voltage to a higher secondary or output voltage through the application of voltage to a first polarized piezoelectric ceramic wafer. The application of voltage to the first piezoelectric wafer generates an extensional stress in that wafer that is mechanically transmitted to a second tightly adhered polarized piezoelectric ceramic wafer, which undergoes a similar and proportional extensional stress and thereby produces an output voltage. The ratio of the first voltage to the second voltage is a function of the piezoelectric properties of the two wafers, the size and geometry of the two wafers and the size and elasticity of the ceramic wafers and other adhesive and pre-stress layers as well as the poling characteristics of the ceramic wafers utilized in the devices described in the above-referenced U.S. Patent Application. Similarly, the resonant frequency of a particular design of such a device will be determined by the same parameters.
Copending U.S. patent application Ser. No. 60/092,284 filed Jul. 10, 1998, which is included by reference thereto, describes a method of manufacturing a piezoelectric transformer by ultrasonically welding together adjacent layers of the transformer.
Copending U.S. patent application Ser. No. 09/177,767 filed Oct. 23, 1998, which is included by reference thereto, describes a method of manufacturing a piezoelectric transformer by co-firing of multiple ceramic layers.
Copending U.S. patent application Ser. No. 09/118,136 filed Jul. 16, 1998, which is included by reference thereto, describes a positive feedback resonant transducer circuit. In the '136 application an output from a step-up transformer is fed back as input to the transformer to produce a twice-stepped-up output; the twice-stepped-up output is fed back as input to the transformer to produce a thrice-stepped-up output. This feedback cycle is repeated (and supplemented by "make-up" power) to generate a multi-stepped up transformer output voltage.
Copending U.S. patent application Ser. No. 60/103,528 filed Oct. 8, 1998, which is included by reference thereto, describes a piezoelectric fluorescent lamp excitation circuit comprising a multi-layer piezoelectric transformer.
It is well known to combine transformers with other circuit elements in the prior art to provide means for converting an input signal of one character (typically at a first voltage) to an output signal of another character (typically at a second voltage). Such "voltage converter" circuits have many applications. Examples of uses for such voltage converter circuits include the conversion of standard 110 VAC house electrical power to DC electrical power at a different voltage, and "inverter" circuits that convert DC electrical signals to AC signals. Certain ballast circuits for gas discharge lamps, such as a fluorescent lamp ballasts, are examples of the latter application.
For reasons discussed above, piezoelectric transformers offer many advantages over prior wire-wound transformers and, therefore, it is desirable to use piezoelectric transformers in voltage converter circuit applications. Prior voltage converter circuits comprising piezoelectric transformer devices are typically of relatively low efficiency and require a relatively high number of circuit components. When used to convert low voltage DC input signals to high voltage AC output signals (such as in gas discharge lamp ballasts), many such prior converter circuits typically rely on the coordinated operation of a plurality of switching transistors (or equivalent electronic components) to alternately apply positive and negative voltage to the transformer, in order to "push" and "pull", respectively, the driver portion of the piezoelectric transformer.
It would be desirable to provide a voltage converter circuit that is highly efficient, that uses fewer circuit components than prior voltage converter circuits that perform comparable signal transformation functions, and that is relatively less expensive to manufacture than prior voltage converter circuits that perform comparable signal transformation functions.
It would also be desirable to provide a solid state voltage converter circuit that has high power transmission capabilities.
It would still further be desirable to be able to provide such a voltage converter circuit to efficiently power and control a gas discharge lamp.
It would still further be desirable to provide a solid state, high power, high efficiency voltage converter circuit in which the transformer output portion of the circuit is electrically isolated from the transformer input portion of the circuit.